Experimental Setups

Inelastic scattering of neutrons

Inelastic scattering of neutrons is an important process that slows down fast neutrons in matter. While elastic scattering on heavy nuclei only causes a comparably small energy loss due to the recoil, the inelastic scattering can lead to energy loss of several hundred keV or more in a single nuclear collision. In addition γ-rays are produced that are an important background e.g. in large detectors used to detect neutrinos or to search for dark matter or exotic decay modes.

The inelastic scattering of neutrons can be studied at nELBE using a double time-of-flight technique, where the scattered neutron and the emitted γ-ray are detected in coincidence, or by measurement of the partial γ-ray production cross sections. The γ-ray angular distribution of 56Fe(n,n’γ) has been investigated with high time-of-flight resolution to allow for a better interpretation of partial γ-ray production cross section measurements [R. Beyer, PhD thesis, TU Dresden 2014, R. Beyer Nuclear Physics A 927 (2014) 41–52].

In the double time-of-flight measurement the emitted γ-ray is detected in an array of BaF2 scintillation detectors and the scattered neutron by an array of plastic scintillators with low detection threshold. The two dimensional distribution of incoming and outgoing neutron time-of-flight allows to kinematically distinguish different excited states in the target nucleus and also multiple inelastic scattering events.

The inelastic scattering cross section measurement is sensitive to the correlation of the neutron and γ-ray emitted in the reaction. A correction for the angular distribution of the γ-ray has been made recently [R. Beyer et al., Proc. Int. Conf. ND 2016].

Neutron-induced fission

Nuclear fission is a complicated large scale motion of atomic nuclei. Even nearly eighty years after its discovery by O. Hahn, F. Strassmann, and L. Meitner there is no complete microscopic understanding of it’s reaction mechanism. The fission process is treated on a complicated multi-dimensional potential energy landscape of the deforming nucleus undergoing fission, which is governed by a complicated interplay of macroscopic and microscopic effects [P. Möller et al. Nature 409 (2001) 785]. Using projectile fragmentation, in-flight studies of many nuclides in the actinide region have become possible [J. Martin et al. Eur. Phys. J. A 51 (2015) 174, K.-H. Schmidt et al. Nuclear Physics A 693 (2001) 169–189]. The GEF model [K.-H. Schmidt, B. Jurado, JEFF-Report 24] has predictive capabilities for all fission observables based on a phenomenological principles, but suffers from similar problems related to the prediction of the fission probability.

The dynamics of the fission process is of fundamental interest for the understanding of quantum systems under the influence of the strong interaction. The fission fragment distributions especially in excitation and kinetic energy are observables that allow conclusion about the dissipation of energy in the nuclear quantum system.

Neutron-induced fission cross sections of actinides such as the Pu-isotopes are of relevance for the development of future nuclear transmutation technologies. Apart from 244Pu, whose abundance in spent nuclear fuel could be neglected, 242Pu is the plutonium isotope with the longest half-life (T1/2 = 373300 a). Thus, there is a special interest to investigate the fission of this particular isotope using fast neutrons. In the TRAKULA project we have developed a novel 242Pu parallel plate fission ionization chamber to measure the neutron induced fission cross section relative to the reference 235U(n,f). The chamber contains eight large area (diameter 76 mm) deposits. The deposits were produced by the nuclear chemistry group of Klaus Eberhardt, at University of Mainz [A. Vascon et al., J Radioanal Nucl Chem (2015) 305:913–919]. They are of unprecedented homogeneity as they were electrodeposited on Si-wafers that have a homogeneous surface.

The 242Pu fission chamber was also used to measure the prompt fission gamma-ray spectrum from spontaneous fission of 242Pu [S. Urlass, Master Thesis, TU Dresden 2017].

The neutron-induced fission cross section of 242Pu was measured relative to 235U using two fission chambers, the H19 transfer instrument from PTB, Braunschweig, and our 242Pu fission chamber containing 37 mg of 242Pu in eight deposits. The neutron beam passes through the fission chambers from the right on the photo. From the measured time-of-flight spectra of the fission events the fission cross section ratio of 242Pu/235U can be determined. The correction for scattering of neutrons in the setup was simulated using MNCP and GEANT4, see [T. Kögler, PhD thesis, TU Dresden 2016].

The time-of-fligh spectra of fission events from 242Pu (red) and 235U (blue). A significant constant background of spontaneous fission events from 242Pu has already been subtracted. The time resolution of the fission chambers has been determined from width of the photofission peak at T = 21 ns and is 2.5 ns (FWHM). The neutron-induced fission rate in the experiment was 6 s-1 for 242Pu and 30 s-1 for 235U.

Relative fission cross section of 242Pu to the reference cross section of 235U in comparison with recent measurements. The systematic uncertainty of our measurements amounts to 3.8 %. The statistical uncertainty is typically below 5 % using a 2 ns time-of-flight binning.

Neutron total cross section measurements

The transmission sample is mounted on a target ladder in front of the neutron collimator. This guarantees that all neutrons reaching the detector have to be traversing the sample and collimator, which is a prerequisite for a transmission measurement. To accomodate for long term drifts of the neutron beam intensity, the sample is computer-controled cycled in and out of the beam in intervals of 10-15 min.

A typical transmission spectrum for a neon sample in a high-pressure gas cell (p = 183 bar, l = 40 cm) shows that the resonances can be fully resolved up to about 3 MeV neutron energy. The total cross section is determined from the measured transmission factor. The time-of-flight dependent dead time correction of the data acquisition system has been reduced to less than 5 % for the target-out measurement by operating the TDC in a continuous measuring mode. The time resolution of the plastic scintillator (0.5 cm thickness) has been improved to 170 ps (1 sigma) [R. Hannaske et al., Eur. Phys. J. A 49 (2013) 137].

Measured transmission function Teff of neon. The time resolution of the plastic scintillator is σt=170 ps.

View of the transmission experiment. A plastic scintillator (0.5 cm x 20 cm) is read out on both ends with high-gain Hamamatsu R2059-1 photomultipliers. To achieve a low detection threshold (ca .10 keV) for neutrons the PMTs are read out in coincidence with the discriminator threshold set below the single photo electron peak.